A polylactic acid-based resin is a resin obtained by polymerizing naturally occurring lactic acid and is a biodegradable resin which can be decomposed by naturally occurring microorganisms. Moreover, it is also excellent in mechanical properties at normal temperatures. It has therefore been attracting public attentions.
A polylactic acid-based resin has been produced generally by polymerizing D-lactic acid and/or L-lactic acid, or ring-opening polymerizing one or two or more lactides selected from the group consisting of L-lactide, D-lactide and DL-lactide.
A polylactic acid-based resin to be obtained varies in physical properties, especially crystallinity, depending upon the content of the D-form component or the L-form component contained in the polylactic acid-based resin. Specifically, the crystallinity of a polylactic acid-based resin to be obtained decreases as the content of the less optical isomer of the D-form component or the L-form component contained in the polylactic acid-based resin increases, and eventually the resin will become amorphous.
As a method for producing a polylactic acid-based resin foam-molded article by causing polylactic acid-based resin foamed particles to foam, in-mold foam-molding has been proposed. The above-mentioned in-mold foam-molding is a method for producing a polylactic acid-based resin foam-molded article having a desired shape in which polylactic acid-based resin foamed particles are filled into a mold, and the polylactic acid-based resin foamed particles are heated with a heat medium, such as hot water or steam, to foam them, and the foamed particles are fusion-bonded to unite together by the foam pressure of the polylactic acid-based resin foamed particles.
Specifically patent document 1 discloses a resin composition prepared by aging under given conditions the resin composition obtained by incorporating a polyisocyanate compound having an isocyanate group of ≧2 equivalents/mol into a polylactic acid having an L-form to D-form molar ratio of from 95/5 to 60/40 or from 40/60 to 5/95, in an amount of 0.5 to 5% by weight based on the polylactic acid, and causing them to react. In addition, it discloses that particles are produced from the aforementioned resin composition, the particles are impregnated with a foaming agent and a foaming aid, the resulting foamable particles are preliminarily foamed to produce preliminarily foamed particles, and the preliminarily foamed particles are filled into a mold and then are foamed to form a molded article with a desired shape.
However, the polylactic acid-based resin constituting the resin composition has a molar ratio of the optical isomer component of the L-form or the D-form, whichever is less, of 5 mol % or more, and the polylactic acid-based resin is low in crystallinity or amorphous and is poor in heat resistance. Therefore, a molded article obtained from this resin composition has insufficient heat resistance, i.e., about 50° C. at most, which has caused problems in practical use.
In the above method, it is considered to use, as the polylactic acid-based resin for constituting the resin composition, a highly crystalline polylactic acid-based resin in which the molar ratio of the less optical isomer of the L-form or the D-form is 5 mol % or less. However, since particles of the above resin composition are impregnated with a foaming agent to form foamable particles and the foamable particles are preliminarily foamed by being heated, crystallization of the polylactic acid-based resin proceeds due to the heat added during the preliminary foaming process. As a result, the preliminarily foamed particles obtained become preliminarily foamed particles with high crystallinity degree, so that the fusion bonding property thereof deteriorates. Therefore, there has been a problem with a molded article obtained using such preliminarily foamed particles that the article is low in mechanical strength due to its low fusion bonding property.
Moreover, although preliminarily foamed particles containing relatively fine cells can be obtained by the above-mentioned method, this method has a problem that the productivity is low because it requires a step of producing particles from a resin composition, and a step of impregnating the particles with a foaming agent and a foaming aid, followed by preliminarily foaming the resulting foamable particles.
Furthermore, patent document 2 discloses a method for continuously producing biodegradable polyester-based resin preliminarily foamed particles wherein in producing preliminarily foamed particles for in-mold molding from a biodegradable polyester-based resin, the biodegradable polyester-based resin and a foaming agent are kneaded with an extruder and extruded in the form of a foamed strand, and the foamed strand is cut to obtain preliminarily foamed particles. It also discloses that as a method for cutting the foamed strand, so-called hot cutting system, in which a strand under foaming is cut while being cooled is preferred (paragraph [0030]).
In the above-mentioned continuous production method, however, since a strand is cut at its portion which has already foamed as described in paragraph [0030], there is a fear that a cut section of the strand may fall into a condition that cut sections of cells are exposed. As a result, the resulting preliminarily foamed particles have had a problem that foaming gas is apt to escape from the cut sections of cells exposed on their surfaces and therefore they are low in foaming property.
Furthermore, preliminarily foamed particles obtained by cutting a strand have another problem that the crystallinity degree of the polyester-based resin constituting the preliminarily foamed particles has increased because they have not been cooled forcibly, and, as a result, the thermal fusion bonding property will be deteriorated.
Patent document 3 discloses a method for producing foamable polylactic acid particles by which foamed particles excellent in moldability and thermal fusion bonding property can be obtained. This production method is a method in which resin particles having a calorific value in differential scanning calorimetry of 15 J/g or more are impregnated with carbon dioxide under particular conditions.
However, since the calorific value in differential scanning calorimetry is 15 J/g or more, the polylactic acid resin is low in crystallinity and is insufficient in heat resistance, i.e., having a heat resisting temperature of about 80° C.
Moreover, since the gas pressure applied when resin particles are impregnated with carbon dioxide is 0.5 to 10 MPa, there has been arisen a problem that crystallization of the polylactic acid proceeds during the impregnation of the resin particles with carbon dioxide, so that the thermal fusion bonding property of the resulting foamable polylactic acid particles is deteriorated and that foamable polylactic acid particles become flat, so that the foaming property thereof will be deteriorated.
Patent document 1: JP-A 2000-17038
Patent document 2: JP-A 2002-302567
Patent document 3: JP-A 2003-73495